The present invention is directed to a paper support which is suitable for use as a base for printed layers and/or varnish layers which are applied to one side of the support. The paper support is especially suitable for use as a base for slide-off decals.
Slide-off decals which are used for decorating articles made of ceramics, porcelain, glass, enamel or plastic, essentially comprise a support of water-permeable paper provided with a barrier layer and a release layer, to which a detachable, cohesive varnish masking film containing the decorative image has been applied. For film-forming, resins comprising acrylate and/or methacrylate polymers are generally employed. The varnish mask also serves to stiffen the more or less thick colorant coatings of the decorative image, which can thus readily be transferred to the article to be decorated. Varnishes comprising acrylate and/or methacrylate polymers are preferably used in those cases in which the slide-off decal, which has been transferred to the article, is later burnt-in, since acrylate and methacrylate polymers are quickly decomposed by baking and do not pass through a liquid stage. As a result, any change or disintegration of the decorative image is prevented.
It has been found that a varnish mask, which comprises the above-described resins and is present on a paper support, tends to develop undesired sticking, for example, under compressive stress, in particular, when it covers a relatively thick, relief-type colorant coating. This area-wide adhesion, which can also be an almost punctiform adhesion in the case of only a few points of contact, of the varnish mask to the reverse side of the sheet which lies on top of the varnish mask in a stack of sheets, is caused both by the weight of the superposed sheets and by solvent residues or plasticizer additions which are retained in the varnish due to temperature and atmospheric humidity.
To prevent the printed and/or varnished sheets from sticking together, it has already been proposed to interpose waxed tissue paper between the individual sheets. Attempts have also been made to prevent the unfavorable adhesion of adjacent sheets by covering the printed layer and/or varnish layer with wax-type products. However, this procedure has the disadvantage that each individual sheet must be coated in an additional process step. Moreover, the problem of mutual adhesion of the sheets cannot be completely overcome by this process. Also, the method of interposing sheets of tissue paper, which results in useful pictures even after prolonged storage, is very expensive and requires several additional process steps, for example, exact interposing and removing of the tissue paper which cannot be used again after removal. In addition, the sheets must be cut up in accordance with the decorative images before use and any superfluous decals remaining on the supports are stored again in a stack, without intermediate tissue-paper sheets. When these decals are to be employed at a later time, they usually stick together and are thus rendered useless.
German Offenlegungsschrift No. 2,551,860 (corresponding to U.S. Pat. No. 4,049,860) discloses a paper support for decals in which the reverse side of the support, i.e. the side of the support which does not have the printed and/or varnish layers, is provided with a water-permeable covering layer containing a polysiloxane and a polyalkylene glycol wax. However, a disadvantage of this support is that the penetration speed of water is slowed by the covering layer so that it takes a longer time for the decorative image to be released by the dissolving release layer. This is particularly unfavorable in an automatic image transfer process, in which the contact time of the slide-off decals with water and the dwell time before image transfer must proceed according to a predetermined cycle. Moreover, the covering layer may increase the blocking resistance of the reverse side of the paper to such an extent that transport problems can arise, in particular, in screen printing machines, where the sheets, which have left the printing station, lie unfixed on conveyor belts and are transported to the dryer at a rising angle. Due to the reduced adhesion, the sheets can then slip on the belts in an uncontrolled manner, thus causing troublesome machine stoppages.